OBJECTIVE:The angle of arteries at bifurcations, as well as the blood flow, are factors of hemodynamic stress on the apical region, where aneurysms often develop. Using images obtained with three-dimensional computed tomographic angiography, we sought to determine the angles between the A1 and A2 segments of the anterior cerebral artery of the anterior communicating artery (ACoA) complex associated with aneurysms. These angles cannot be detected by conventional cerebral angiography.METHODS:The course of the anterior cerebral artery was studied using three-dimensional computed tomographic angiography in 42 consecutive patients with ACoA aneurysms. Twenty-one other subjects, randomly chosen from patients without aneurysms, served as controls. Bilateral A1–A2 angles of the contrast-opacified anterior cerebral artery were measured by three-dimensional computed tomographic angiography in patients with normoplastic A1 segments, and the relationship between the angle and the association of aneurysms was analyzed using cerebral angiography.RESULTS:Of the 42 patients with ACoA aneurysms, 19 patients showed hypo- or aplastic A1 segments, as did only 2 of the 21 patients without ACoA aneurysms. The average A1–A2 angle was determined to be 116 ± 24 degrees (mean ± standard deviation) in 18 patients having ACoA complexes with normoplastic A1 segments with aneurysms; 17 patients without aneurysms had A1–A2 angles measuring 143 ± 14 degrees (P< 0.0001). The A1–A2 angle associated with ACoA aneurysms was 103 ± 20 degrees, which was much smaller than that of the non-aneurysm side in the former group (128 ± 20 degrees) (P= 0.0036).CONCLUSION:ACoA aneurysms are associated with the smaller A1–A2 angle junction of the ACoA complex, where higher hemodynamic stress may occur in patients with normoplastic A1 segments.